The invention concerns underwater tracking by acoustic waves, and more particularly transmitters carried by equipment or mobile units to be tracked and possibly to be identified. It also concerns a method and a facility to which such transmitters apply.
Various techniques are used in underwater tracking. In general, they use the propagation of acoustic waves.
A first method consists of providing the mobile unit to be tracked with a transceiver. At a regular rate the mobile unit interrogates beacons positioned on the bottom of the ocean. The measurement of the round trip propagation time of the acoustic waves, with a known velocity of sound in water, makes it possible to calculate the mobile unit-beacon distances. The coordinates of the beacons being known, the mobile unit is then tracked at the intersection of spheres centered on the beacons and radii equal to the propagation times divided by twice the velocity and corrected, if necessary, by a possible delay. This well-known technique is called tracking by the xe2x80x9clong-baseline system.xe2x80x9d
Higher performance systems are used by military testing centers. These are torpedo trajectory tracking systems, for example. They are fixed or mobile. In both cases, the points of reception of the acoustic signals are hydrophones located either close to the bottom of the ocean, or suspended from buoys. THOMSON-CSF""s European patent No. 0 243 240 describes such a device. The mobile units whose trajectory is to be tracked are provided with acoustic transmitters. Periodically, every 500 ms., for example, they transmit pulse strings chosen from between two frequencies (a technique called SFSK [Sinusoidal Frequency Shift Keying] in English terminology).
Other transmitters used in trajectory tracking transmit a first pulse at a rate of 1 Hz, followed by a pulse offset by a constant factor and of a size proportional to the depth of the mobile unit. These transmitters can have a clock locked on to an external time reference prior to the launching of the mobile unit. They are then called synchronous.
Also known are acoustic transmitters designed to locate the black boxes of aircraft sunk in the ocean. They transmit pure 37.5 kHz pulses which, although they make it possible to be guided toward the source, do not enable them to be identified. Indeed, the box""s transmitter containing the voice recordings transmits the same signals as the box containing the information relative to the flight""s parameters. This results in a complication of the search and recovery operations.
Existing transmitters have numerous limitations, especially when numerous mobile units to be tracked by trajectory are present simultaneously in the water. This is the case particularly for a group of underwater divers. In effect, the simultaneous reception, by the same hydrophone, of acoustic pulses from different mobile units poses detection problems. Included among those problems but not limited hereto are: the poor adjustment of the automatic gain control loop of the preamplifiers, wave interference phenomena that result from fading or a glut of signals.
Moreover, not only the interference between the direct waves should be taken into consideration, but also interference with multiple trajectories frequently encountered in shallow and medium-depth waters. They are related to the multiple reflections of the signals on the bottom and surface of the ocean.
Consider, for example, tracking a team of 2, 4 or 8 divers by using a device involving four buoys located at the surface, stabilized by anchoring, in a square configuration of 1000 meters per side. The divers are fitted with conventional transmitters each of which transmits on its own frequency two acoustic pulses, one synchronous with a time reference (the GPS time, for example), the other making it possible to encode his depth according to the procedure described previously. For example, when this pulse is transmitted with a delay of 45 milliseconds, it corresponds to a depth of 30 meters. The following table presents the probability of detecting without interference the first transmission of a given diver on at least three buoys, which is the minimum number required to locate the diver without ambiguity using X-Y coordinates.
Columns show different grid cell sizes.
A grid cell of 500 m means that the divers are randomly distributed in a volume of 500 mxc3x97500 m, with a depth of between 20 and 40 meters.
In the calculation, the multiple trajectory phenomena are assumed to create interference for a duration of 10 milliseconds following the end of the transmission of an acoustic pulse of 5 milliseconds duration.
The table above shows that it is very difficult to locate divers, at a high rate, when they are grouped together inside a small volume and use synchronous transmitters that have an identical transmission recurrence for all of the divers.
In general, the problem posed consists of making it possible to track and identify a set of underwater objects or mobile units, and do so accurately, without ambiguity, and regardless of the position of the mobile units with respect to each other and with respect to the detection hydrophones. Moreover, the cost of manufacturing these transmitters should be low.
The acoustic tracking transmitters covered by the present invention are characterized by the fact that they have electronic circuits that generate aperiodic pulsed acoustic signals, the transmission dates of which are distributed according to a known, predetermined pseudo-random sequence, both at the transmission equipment itself, as well as at the receiving equipment or processing center.
According to another characteristic, said electronic circuits are configured so that the aperiodic pseudo-random sequence of pulses they generate is a sequence of signals encoded by hops in wide or narrow frequency bands, according to an order that is predetermined and known, both at the transmission equipment itself, as well as at the receiving equipment or processing center.
The facility according to the invention is notable by the fact that it comprises:
transmitters suitable for generating a periodic pulsed acoustic signals, integrated into equipment or mobile units to be tracked and/or identified;
one or more points of measurement provided with an acoustic receiver, a time scale reference and a means of tracking said point of measurement;
and one or more processing centers that make it possible to identify the transmitters by level 2 correlation calculations between a series of moments of reception of pulses and the pseudo-random sequences peculiar to each transmitter.
Alternatively, the point(s) of measurement can be fixed and previously tracked by any means of positioning in a local or geographic referential system.
The method of the invention is notable in that the equipment or mobile units to be tracked and/or identified are equipped with acoustic transmitters transmitting aperiodic pulsed acoustic signals according to a predetermined pseudo-random sequence; in the work area where the equipment or mobile units to be tracked are located, one or more points of measurement are deployed, each of which points is provided with an acoustic receiver, a time scale reference and a means of tracking said point of measurement; the information produced by the points of measurement is transmitted to a processing center where, at each moment of detection on a given frequency band, it is associated with a precise dating in a time scale referential system and a position of said point of measurement in a spatial referential system; all of this information from different points of measurement is collected at said processing center so that it can be processed by level 2 correlation with the pseudo-random reference sequences pertaining to each of the transmitters being sought.
Alternatively, the level 2 correlation processing can be done at the point(s) of measurement.
As will be seen further on, in the description of one form of embodiment, one particular solution to the problem posed consists of having each mobile unit transmit cyclically a series of pulsed and aperiodic acoustic signals.